Heat Deflection Temperature

Above-room temperature at which a polymeric material deflects by a specific amount under a specific flexural load. Useful for setting approximate boundaries on elevated service temperature, for applications where stiffness is important. Not measured for metals or ceramics.

The terms heat deflection (or distortion) temperature (HDT) and deflection temperature under load (DTUL) are synonymous. Testing is standardized under ASTM D648 and ISO 75. The two standards are generally considered equivalent, but they are not identical.

Both standards allow testing at two loading levels: 1.82 MPa or 264 psi, and 455 kPa or 66 psi. Under ISO, the two levels are called Method A and Method B, respectively. The ASTM test limits itself to just the two levels, while the ISO also has a third level (Method C, at 8 MPa). This third level is rarely used.